Rotary pump with exclusively hydrodynamically suspended impeller

ABSTRACT

A pump assembly  1, 33, 200  adapted for continuous flow pumping of blood. In a particular form the pump  1, 200  is a centrifugal pump wherein the impeller  100, 204  is entirely sealed within the pump housing  2, 201  and is exclusively hydrodynamically suspended therein as the impeller rotates within the fluid  105  urged by electromagnetic means external to the pump cavity  106, 203.    
     Hydrodynamic suspension is assisted by the impeller  100, 204  having deformities therein such as blades  8  with surfaces tapered from the leading edges  102, 223  to the trailing edges  103, 224  of bottom and top edges  221, 222  thereof.

FIELD OF THE INVENTION

[0001] This invention relates to rotary pumps adapted, but notexclusively, for use as artificial hearts or ventricular assist devicesand, in particular, discloses in preferred forms a seal-less shaft-lesspump featuring open or closed (shrouded) impeller blades with at leastparts of the impeller used as hydrodynamic thrust bearings and withelectromagnnetic torque provided by the interaction between magnetsembedded in the blades or shroud and a rotating current patterngenerated in coils fixed relative to the pulp housing.

BACKGROUND ART

[0002] This invention relates to the art of continuous or pulsatile flowrotary pumps and, in particular, to electrically driven pumps suitablefor use although not exclusively as an artificial heart or ventricularassist device. For permanent implantation in a human patient, such pumpsshould ideally have the following characteristics: no leakage of fluidsinto or from the bloodstream; parts exposed to minimal or no wear;minimum residence time of blood in pump to avoid thrombosis (clotting);minimum shear stress on blood to avoid blood cell damage such ashaexmolysis; maximum efficiency to maximise battery duration andminimise blood heating; and absolute reliability

[0003] Several of these characteristics are very difficult to meet in aconventional pump configuration including a seal, i.e. with an impellermounted on a shaft which penetrates a wall of the pumping cavity, asexemplified by the blood pumps referred to in U.S. Pat. No. 3,957,389 toRafferty et al., U.S. Pat. No. 4, 625,712 to Wampler, and U.S. Pat. No.5,275,580 to Yamazaki. Two main disadvantages of such pumps are firstlythat the seal needed on the shaft may leak, especially after wear, andsecondly that the rotor of the motor providing the shaft torque remainsto be supported, with mechanical bearings such as ball-bearingsprecluded due to wear. Some designs, such as U.S. Pat. No. 4,625,712 toWampler and U.S. Pat. No. 4,908,012 to Moise et al., have overcome theseproblems simultaneously by combining the seal and the bearing into onehydrodynamic bearing, but in order to prevent long residence times theyhave had to introduce means to continuously supply a blood-compatiblebearing purge fluid via a percutaneous tube.

[0004] In seal-less designs, blood is permitted to flow through the gapin the motor, which is usually of the brushless DC type, i.e. comprisinga rotor including permanent magnets and a stator in which an electriccurrent pattern is made to rotate synchronously with the rotor. Suchdesigns can be classified according to the means by which the rotor issuspended: contact bearings, magnetic bearings or hydrodynamic bearings,though some designs use two of these means.

[0005] Contact or pivot bearings, as exemplified by U.S. Pat. No.5,527,159 to Bozeman et al. and U.S. Pat. No. 5,399,074 to Nose et al.,have potential problems due to wear, and cause very high localisedheating and shearing of the blood, which can cause deposition anddenaturation of plasma proteins, with the risk of embolisation andbearing seizure.

[0006] Magnetic bearings, as exemplified by U.S. Pat. No. 5,350,283 toNakazeki et al., U.S. Pat. No. 5,326,344 to Bramm et al. and U.S. Pat.No. 4,779,614 to Moise et al., offer contactless suspension, but requirerotor position measurement and active control of electric current forstabilisation of the position in at least one direction, according toEarnshaw's theorem. Position measurement and feedback control introducesignificant complexity, increasing the failure risk. Power use by thecontrol current implies reduced overall efficiency. Furthermore, size,mass, component count and cost are all increased.

[0007] U.S. Pat. No. 5,507,629 to Jarvik claims to have found aconfiguration circumventing Earnshaw's Theorem and thus requiring onlypassive magnetic bearings, but this is doubtful and contact axialbearings are included in any case. Similarly, passive radial magneticbearings and a pivot point are employed in U.S. Pat. No. 5,443,503 toYamane.

[0008] Prior to the present invention, pumps employing hydrodynamicsuspension, such as U.S. Pat. No. 5,211,546 to Isaacson et al. and U.S.Pat. No. 5,324,177 to Golding et al., have used journal bearings, inwhich radial suspension is provided by the fluid motion between twocylinders in relative rotation, an inner cylinder lying within andslightly off axis to a slightly larger diameter outer cylinder. Axialsuspension is provided magnetically in U.S. Pat. No. 5,324,177 and byeither a contact bearing or a hydrodynamic thrust bearing in U.S. Pat.No. 5,211,546.

[0009] A purging flow is needed through the journal bearing, a highshear region, in order to remove dissipated heat and to prevent longfluid residence time. It would be inefficient to pass all the fluidthrough the bearing gap, of small cross-sectional area, as this woulddemand an excessive pressure drop across the bearing. Instead a leakagepath is generally provided from the high pressure pump outlet, thoughthe bearings and back to the low pressure pump inlet, implying a smallreduction in outflow and pumping efficiency. U.S. Pat. No. 5,324,177provides a combination of additional means to increase the purge flow,namely helical grooves in one of the bearing surfaces, and a smalladditional set of impellers.

[0010] U.S. Pat. No. 5,211,546 provides 10 embodiments with variouslocations of cylindrical bearing surfaces. One of these embodiments, thethird, features a single journal bearing and a contact axial bearing.

[0011] Embodiments of the present invention offer a relatively low costand/or relatively low complexity means of suspending the rotor of aseal-less blood pump, thereby overcoming or ameliorating the problems ofexisting devices mentioned above.

SUMMARY OF THE INVENTION

[0012] According to one aspect of the present invention, there isdisclosed a rotary blood pump for use in a heart assist device or likedevice, said pump having an impeller suspended in use within a pumphousing exclusively by hydrodynamic thrust forces generated by relativemovement of said impeller with respect to and within said pump housings.

[0013] Preferably at least one of said impeller or said housing includesat least one deformed surface which, in use, moves relative to a facingsurface on the other of said impeller or said housing thereby to cause arestriction in the form of a reducing distance between the surfaces withrespect to the relative line of movement of said deformed surfacethereby to generate relative hydrodynamic thrust between said impellerand said housing which includes everywhere a localized thrust componentsubstantially and everywhere normal to the plane of movement of saiddeformed surface with respect to said facing surface.

[0014] Preferably the combined effect of the localized normal forcesgenerated on the surfaces of said impeller is to produce resistiveforces against movement in three translational and two rotationaldegrees of freedom thus supporting the impeller for rotational movementwithin said housing exclusively by hydrodynamic forces.

[0015] Preferably said thrust forces are generated by blades of saidimpeller.

[0016] More preferably said thrust forces are generated by edges of saidblades of said impeller.

[0017] Preferably said edges of said blades are tapered or non-planar sothat a thrust is created between the edges and the adjacent pump casingduring relative movement therebetween.

[0018] Preferably said edges of said blades are shaped such that the gapat the leading edge of the blade is greater than at the trailing edgeand thus the fluid which is drawn through the gap experiences a wedgeshaped restriction which generates a thrust.

[0019] Preferably the pump is of centrifugal type or mixed flow typewith impeller blades open on both front and back faces of the pumphousing.

[0020] Preferably the front face of the housing is made conical, inorder that the thrust perpendicular to the conical surface has a radialcomponent, which provides a radial restoring force to a radialdisplacement of the impeller axis during use.

[0021] Preferably the driving torque of said impeller derives from themagnetic interaction between permanent magnets within the blades of theimpeller and oscillating currents in windings encapsulated in the pumphousing.

[0022] Preferably said blades include magnetic material therein, themagnetic material encapsulated within a biocompatible shell or coating.

[0023] Preferably said biocompatible shell or coating comprises adiamond coating or other coating which can be applied at lowtemperature.

[0024] Preferably internal walls of said pump which can come intocontact with said blades during use are coated with a hard material suchas titanium nitride or diamond coating.

[0025] Preferably said impeller comprises an upper conical shroud havingsaid taper or other deformed surface therein and wherein blades of saidimpeller are supported below said shroud.

[0026] Preferably said impeller further includes a lower shroud mountedin opposed relationship to said upper conical shroud and whereas saidblades are supported within said upper and said lower shroud.

[0027] Preferably said deformed surface is located on said impeller.

[0028] Preferably said deformed surface is located within said housing.

[0029] Preferably forces imposed on said impeller in use, other thanhydrodynamic forces, are controlled by design so that, over apredetermined range of operating parameters, said hydrodynamic thrustforces provide sufficient thrust to maintain said impeller suspended inuse within said pump housing.

[0030] Preferably at least one face of the housing is made conical, inorder that the thrust perpendicular to it has a radial component, whichprovides a radial restoring force to a radial displacement of theimpeller axis. Similarly, an axial displacement toward either the frontor the back face increases the thrust from that face and reduces thethrust from the other face. Thus the sum of the forces on the impellerdue to inertia (within limits), gravity and any bulk radial or axialhydrodynamic force on the impeller can be countered by a restoring forcefrom the thrust bearings after a small displacement of the impellerwithin the housing relative to the housing in either a radial or axialdirection.

[0031] In a preferred embodiment, the impeller driving torque derivesfrom the magnetic interaction between permanent magnets within theblades of the impeller and oscillating currents in windings encapsulatedin the pump housing.

[0032] In a further broad form of the invention there is provided arotary blood pump having an impeller suspended exclusivelyhydrodynamically by thrust forces generated by the impeller duringmovement in use of the impeller.

[0033] Preferably said thrust forces are generated by blades of saidimpeller or by deformities therein.

[0034] More preferably said thrust forces are generated by edges of saidblades of said impeller.

[0035] Preferably said edges of said blades are tapered.

[0036] In an alternative preferred form said pump is of axial type.

[0037] Preferably within a uniform cylindrical section of the pumphousing, tapered blade edges form a radial hydrodynamic bearing.

[0038] In a further broad form of the invention there is provided arotary blood pump having a housing within which an impeller acts byrotation about an axis to cause a pressure differential between an inletside of a housing of said pump and an outlet side of the housing of saidpump; said impeller suspended exclusively hydrodynamically by thrustforces generated by the impeller during movement in use of the impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Embodiments of the present invention will now be described, withreference to the accompanying drawings, wherein:

[0040]FIG. 1 is a longitudinal cross-sectional view of a preferredembodiment of the invention;

[0041]FIG. 2 is a cross-sectional view taken generally along the lineZ-Z of FIG. 1;

[0042]FIG. 3A is a cross-sectional view of an impeller blade takengenerally along the line A-A of FIG. 2;

[0043]FIG. 3B is an enlargement of the blade-pump housing interfaceportion of FIG. 3A;

[0044]FIG. 3C is an alternative impeller blade shape;

[0045]FIGS. 4A, B, C illustrate various possible locations of magnetmaterial within a blade;

[0046]FIGS. 5A, B and C are left-hand end views of possible windinggeometries taken generally along the line S-S of FIG. 1;

[0047]FIG. 6 is a diagrammatic cross-sectional view of an alternativeembodiment of the invention as an axial pump;

[0048]FIG. 7 is an exploded, perspective view of a centrifugal pumpassembly according to a further embodiment of the invention;

[0049]FIG. 8 is a perspective view of the impeller of the assembly ofFIG. 7;

[0050]FIG. 9 is a perspective, cut away view of the impeller of FIG. 8within the pump assembly of FIG. 7;

[0051]FIG. 10 is a side section indicative view of the impeller of FIG.8;

[0052]FIG. 11 is a detailed view in side section of edge portions of theimpeller of FIG. 10;

[0053]FIG. 12 is a block diagram of an electronic driver circuit for thepump assembly of FIG. 7;

[0054]FIG. 13 is a graph of head versus flow for the pump assembly ofFIG. 7;

[0055]FIG. 14 is a graph of pump efficiency versus flow for the pumpassembly of FIG. 7;

[0056]FIG. 15 is a graph of electrical power consumption versus flow forthe pump assembly of FIG. 7,

[0057]FIG. 16 is a plan, section view of the pump assembly showing avolute arrangement according to a preferred embodiment;

[0058]FIG. 17 is a plan, section view of a pump assembly showing analternative volute arrangement;

[0059]FIG. 18 is a plan view of an impeller according to a furtherembodiment of the invention;

[0060]FIG. 19 is a plan view of an impeller according to a furtherembodiment of the invention;

[0061]FIG. 20 is a perspective view of an impeller according to afurther embodiment of the invention;

[0062]FIG. 21 is a perspective view of an impeller according to yet afurther embodiment of the invention;

[0063]FIG. 22 is a perspective, partially cut away view of an impelleraccording to yet a further embodiment of the invention;

[0064]FIG. 23 is a top, perspective view of the impeller of FIG. 22;

[0065]FIG. 24 is a perspective view of the impeller of FIG. 22 with itstop shroud removed;

[0066]FIG. 25 illustrates an alternative embodiment wherein the deformedsurface is located on the pump housing; and

[0067]FIG. 26 illustrates a further embodiment wherein deformed surfacesare located both on the impeller and on the housing.

[0068]FIG. 27 illustrates diagramatically the basis of operation of the“deformed surfaces” utilised for hydrodymanic suspension of embodimentsof the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0069] The pump assemblies according to various preferred embodiments tobe described below all have particular, although not exclusive,application for implantation in a mammalian body so as to at leastassist, if not take over, the function of the mammalian heart. Inpractice this is performed by placing the pump assembly entirely withinthe body of the mammal and connecting the pump between the leftventricle and the aorta so as to assist left side heart function. It mayalso be connected to the right ventricle and pulmonary artery to assistthe right side of the heart.

[0070] In this instance the pump assembly includes an impeller which isfully sealed within the pump body and so does not require a shaftextending through the pump body to support it. The impeller issuspended, in use, within the pump body by the operation of hydrodynamicforces imparted as a result of the interaction between the rotatingimpeller, the internal pump walls and the fluid which the impellercauses to be urged from an inlet of the pump assembly to an outletthereof.

[0071] A preferred embodiment of the invention is the centrifugal pump1, as depicted in FIGS. 1 and 2, intended for implantation into a human,in which case the fluid referred to below is blood. The pump housing 2,can be fabricated in two parts, a front part 3 in the form of a housingbody and a back part 4 in the form of a housing cover, with a smoothjoin therebetween, for example at 5 in FIG. 1. The pump 1 has an axialinlet 6 and a tangential outlet 7. The rotating part 100 is of verysimple form, comprising only blades 8 and a blade support 9 to holdthose blades fixed relative to each other. The blades may be curved asdepicted in FIG. 2, or straight, in which case they can be either radialor back-swept, i.e. at an angle to the radius. This rotating part 100will hereafter be called the impeller 100, but it also serves as abearing component and as the rotor of a motor configuration as to hefurther described below whereby a torque is applied by electromagneticmeans to the impeller 100. Note that the impeller has no shaft and thatthe fluid enters the impeller from the region of its axis RR. Some ofthe fluid passes in front of the support cone 9 and some behind it, sothat the pump 1 can be considered of two-sided open type, as compared toconventional open centrifugal pumps, which are only open on the frontside. Approximate dimensions found adequate for the pump 1 to perform asa ventricular assist device, when operating at speeds in the range 1,500rpm to 4,000 rpm, are outer blade diameter 40 mm, outer housing averagediameter 60 mm, and housing axial length 40 mm.

[0072] As the blades 8 move within the housing, some of the fluid passesthrough the gaps, much exaggerated in FIGS. 1 and 3, between the bladeedges 101 and the housing front face 10 and housing back face 11. In allopen centrifugal pumps, the gaps are made small because this leakageflow lowers the pump hydrodynamic efficiency. In the pump disclosed inthis embodiment, the gaps are made slightly smaller than is conventionalin order that the leakage flow chain be utilised to create ahydrodynamic bearing. For the hydrodynamic forces to be sufficient, theblades may also be tapered as depicted in FIGS. 3A and 3B, so that thegap 104 is larger at the leading edge 102 of the blade 8 than at thetrailing edge 103 thereby providing one example of a “deformed surface”as described elsewhere in this specification. The fluid 105 which passesthrough the gap thus experiences a wedge shaped restriction whichgenerates a thrust, as described in Reynolds' theory of lubrication(see, for example, “Modern Fluid Dynamics, Vol. 1 Incompressible Flow”,by N. Curle and H. J. Davies, Van Nostrand, 1968). For bladesconsiderably thinner than their axial length, the thrust is proportionalto the square of the blade thickness at the edge, and thus in thisembodiment thick blades are favoured, since if the proportion of thepump cavity filled by blades is constant, then the net thrust force willbe inversely proportional to the number of blades. However, the bladeedges can be made to extend as tails from thin blades as depicted inFIG. 3C in order to increase the blade area adjacent the walls.

[0073] In one particular form, the tails join adjacent blades so as toform a complete shroud with wedges or tapers incorporated therein. Anexample of a shroud design as well as other variations on the bladestructure will be described later in this specification.

[0074] For manufacturing simplicity, the housing front face 10 can bemade conical, with an angle of around 45° so that it provides both axialand radial hydrodynamic forces. Other angles are suitable that achievethe functional requirements of this pump including the requirements forboth axial and radial hydrodynamic forces.

[0075] Other curved surfaces are possible provided both axial and radialhydrodynamic forces can be produced as a result of rotation of theblades relative to the housing surfaces.

[0076] The housing back face 11 can include a roughly conical extension12 pointing into the pump cavity 106, to eliminate or minimise theeffect of the flow stagnation point on the axis of the back housing.

[0077] Alternatively extension 12 can resemble an impeller eye to makethe flow mixed.

[0078] In this preferred embodiment, for manufacturing simplicity andfor uniformity in the flow axial direction RR, the housing back face 11is made flat over the bearing surfaces, i.e. under the blade edges. Withthis the case, a slacker tolerance on the alignment between the axes ofthe front part 3 and back part 4 of the housing 2 is permissible. Analternative is to make the back face 11 conical at the bearing surfaces,with taper in the opposite direction to the front face 10, so that thehydrodynamic forces from the back face will also have radial components.Tighter tolerance on the axes alignment would then be required, and someof the flow would have to undergo a reversal in its axial direction.Again a roughly conical extension (like 12) will be needed. There may besome advantage in making the housing surfaces and blade edgesnon-straight, with varying tangent angle, although this will imposegreater manufacturing complexity.

[0079] There are several options for the shape of the taper, but in thepreferred embodiment the amount of material removed simply varieslinearly or approximately linearly across the blade. For the back face,the resulting blade edges are then planes at a slight inclination to theback face. For the front face, the initial blade edges are curved andthe taper only removes a relatively small amount of material so theystill appear curved. Alternative taper shapes can include a step in theblade edge, though the corner in that step would represent a stagnationline posing a thrombosis risk.

[0080] For a given minimum gap, at the trailing blade edge, thehydrodynamic force is maximal if the gap at the leading edge isapproximately double that at the trailing edge. Thus the taper, whichequals the leading edge gap minus the trailing edge gap, should bechosen to match a nominal minimum gap, once the impeller has shiftedtowards that edge. Dimensions which have been found to give adequatethrust forces are a taper of around 0.05 mm for a nominal minimum gap ofaround 0.05 mm, and an average circumferential blade edge thickness ofaround 6 mm for 4 blades. For the front face, the taper is measuredwithin the plane perpendicular to the axis. The axial length of thehousing between the front and back faces at any position should then bemade about 0.2 mm greater than the axial length of the blade, when it iscoaxial with the housing, so that the minimum gaps are both about 0.1 mmaxially when the impeller 100 is centrally positioned within the housing2. Then, for example, if the impeller shifts axially by 0.05 mm, theminimum gaps will he 0.05 mm at one face and 0.15 mm at the other, face.The thrust increases with decreasing gap and would be much larger fromthe 0.05 mm gap than from the 0.15 mm gap, about 14 times larger for theabove dimensions. Thus there is a net restoring force away from thesmaller gap.

[0081] Similarly, for radial shifts of the impeller the radial componentof the thrust from the smaller gap on the conical housing front facewould offer the required restoring radial force, The axial component ofthat force and its torque on the impeller would have to be balanced byan axial force and torque from the housing back face, and so theimpeller will also have to shift axially and tilt its axis to be nolonger parallel with the housing axis. Thus as the person moves and thepump is accelerated by external forces, the impeller will continuallyshift its position and alignment, varying the gaps in such a way thatthe total force and torque on the impeller 100 match that demanded byinertia. The gaps are so small, however, that the variation inhydrodynamic efficiency will be small, and the pumping action of theblades will be approximately the same as when the impeller is centrallylocated.

[0082] While smaller gaps imply greater hydrodynamic efficiency andgreater bearing thrust forces, smaller gaps also demand tightermanufacturing tolerances, increase frictional drag on the impeller, andimpose greater shear stress an the fluid. Taking these points in turn,for the above 0.05 mm tapers and gaps, tolerances of around 0.005 mm areneeded, which imposes some cost penalty but is achievable. A tightertolerance is difficult, especially if the housing is made of a plastic,given the changes in dimension caused by temperature and possibleabsorption of fluid by plastic materials which may be in contact withthe blood such as Acrylic of polyurethane. The frictional drag for theabove gaps produces much smaller torque than the typical motor torque.Finally, to estimate the shear stress, consider a rotation speed of3,000 rpm and a typical radius of 15 mm, at which the blade speed is 4.7ms⁻¹ and the average velocity shear for an average gap of 0.075 mm is6.2×10⁴ s³¹ . For blood of dynamic viscosity 3.5×10⁻³ kgm⁻¹s⁻¹, theaverage shear stress would be 220 Nm⁻². Other prototype centrifugalblood pumps with closed blades have found that slightly larger gaps,e.g. 0.15 mm, are acceptable for haemolysis. A major advantage of theopen blades of the present invention is that a fluid element that doespass through a blade edge gap will have very short residence time inthat gap, around 2×10⁻³ S, and the fluid element will most likely beswept though the pump without passing another blade edge.

[0083] With particular reference to FIG. 3A and 3B typical workingclearances and working movement for the impeller 8 with respect to theupper and lower housing surfaces 10, 11 is of the order of 100 micronsclearance at the top and at the bottom. In use gravitational and otherforces will bias the impeller 8 closer to one or other of the housingwalls resulting, typically in a clearance at one interface of the orderof 50 microns and a corresponding larger clearance at the otherinterface of the order of 150 microns. In use, likely maximum practicalclearances will range from 300 microns down to 1 micron.

[0084] Typical restoring forces for a 25 gram rotor mass spinning at 220rpm are 1.96 Newtons at a 20 micron clearance extending to 0.1 Newton atan 80 micron clearance.

[0085] To minimise the net force required of the hydrodynamic bearings,the net axial and radial hydrodynamic forces on the impeller from thebulk fluid flow should be minimised, where “bulk” here means other thanfrom the bearing thrust surfaces.

[0086] The radial force on the impeller depends critically on the shapeof the output flow collector or volute 13. The shape should be designedto minimise the radial impeller force over the desired range of pumpspeeds, without excessively lowering the pump efficiency. The optimalshape will have a roughly helical perimeter between the “cutwater” andoutlet. The radial force can also be reduced by the introduction of aninternal division in the volute 13 to create a second output flowcollector passage, with tongue approximately diametrically opposite tothe tongue of the first passage.

[0087] An indicative plan view of impeller 100 relative to housing 2 isshown in FIG. 2 having a concentric volute 13.

[0088]FIG. 17 illustrates the alternative volute arrangement comprisinga split volute created by volute barrier 107 which causes volute 108 ina first hemisphere of the housing 2 to split into first half volute 109and second half volute 110 over the second hemisphere. The hemispheresare defined respectively on each side of a diameter of the housing 2which passes through or near exit point 111 of outlet 7.

[0089] In alternative forms concentric volutes can be utilised,particularly where specific speed is relatively low.

[0090] In a further particular form a vaneless diffuser may also reducethe radial force.

[0091] In regard to the bulk hydrodynamic axial force, if the bladecross-section is made uniform in the axial direction along therotational axis, apart from the conical front edge, then the pressureacting on the blade surface (excluding the bearing edges) will have noaxial component. This also simplifies the blade manufacture. The bladesupport cone 9 must then be shaped to minimise axial thrust on theimpeller and minimise disturbance to the flow over the range of speeds,while maintaining sufficient strength to prevent relative blademovement. The key design parameter affecting the axial force is theangle of the cone. The cone is drawn in FIG. 1 as having the sameinternal diameter as the blades, which may aid manufacture. However, thecone could be made with larger or smaller internal diameter to theblades. There may be advantage in using a non-axisymmetric support“cone”, e.g. with larger radius on the trailing surface of a blade thanthe radius at the leading surface of the next blade. If the blades aremade with non-uniform cross-section to increase hydrodynamic efficiency,then any bulk hydrodynamic axial force on them can be balanced byshaping the support cone to produce an opposite bulk hydrodynamic axialforce on it.

[0092] Careful design of the entire pump, employing computational fluiddynamics, is necessary to determine the optimal shapes of the blades 8,the volute 13, the support cone 9 and the housing 2, in order tomaximise hydrodynamic efficiency while keeping the bulk fluidhydrodynamic forces, shear and residence times low. All edges and thejoins between the blades and the support cone should be smoothed.

[0093] The means of providing the driving torque on the impeller 100 ofthe preferred embodiment of the invention is to encapsulate permanentmagnets 14 in the blades 8 of the impeller 100 and to drive them with arotating magnetic field pattern from oscillating currents in windings 15and 16, fixed relative to the housing 2. Magnets of high remanence suchas sintered rare-earth magnets should be used to maximize motorefficiency. The magnets can be aligned axially but greater motorefficiency is achieved by tilting the magnetisation direction to anangle of around 15° to 30° outwards from the inlet axis, with 22.5° tiltsuitable for a body of conical angle 45° The magnetisation directionmust alternate in polarity for adjacent blades. Thus there must be aneven number of blades. Since low blade number is preferred for thebearing force, and since two blades would not have sufficient bearingstiffness to rotation about all axis through the blades andperpendicular to the pump housing (unless the blades are very curved),four blades are recommended. A higher number of blades, for example 6 or8 will also work.

[0094] Some possible options for locating the magnets 14 within theblades 8 are shown in FIG. 4. The most preferred which is depicted inFIG. 4A, is for the blade to be made of magnet material apart from abiocompatible shell or coating to prevent fluid corroding the magnetsand to prevent magnet material (which may be toxic) entering the bloodstream. The coating should also be sufficiently durable especially atblade corners to withstand rubbing during start-up or during inadvertentbearing touch down.

[0095] In one particular form the inside walls of the pump housing 2 arealso coated with a biologically compatible and wear resistant materialsuch as diamond coating or titanium nitride so that wear on both of thetouching surfaces is minimised.

[0096] An acceptable coating thickness is approximately 1 micron.

[0097] A suitable impeller manufacturing method is to die-press theentire impeller, blades and support cone, as a single axially alignedmagnet. The die-pressing is much simplified if near axially uniformblades are used (blades with an overhang such as in FIG. 3C areprecluded). During pressing, the crushed rare-earth particles must bealigned in an axial magnetic field. This method of die-press withparallel alignment direction is cheaper for rare-earth magnets, althoughit produces slightly lower remanence magnets. The tolerance indie-pressing is poor, and grinding of the tapered blade edges isrequired. Then the magnet impeller can be coated, for example byphysical vapour deposition, of titanium nitride for example, or bychemical vapour deposition, of a thin diamond coating or a tefloncoating.

[0098] In an alternative form the magnet material can be potted intitanium or a polymeric housing which is then, in turn, coated with abiologically compatible and tough material such as diamond coating ortitanium nitride.

[0099] Finally, to create the alternating blade polarity the impellermust be placed in a special pulse magnetisation fixture, with anindividual coil surrounding each blade. The support cone of adie-pressed magnet impeller acquires some magnetisation near the blades,with negligible influence.

[0100] Alternative magnet locations are sketched in FIG. 4B and FIG. 4Cin which quadrilateral or circular cross-section magnets 14 are insertedinto the blades. Sealing and smoothing of the blade edges over theinsertion holes is then required to reinstate the taper.

[0101] All edges in the pump should be radiused and surfaces smoothed toavoid possible damage to formed elements of the blood.

[0102] The windings 15 and 16 of the preferred embodiment are slotlessor air-gap windings with the same pole number as the impeller, namelyfour poles in the preferred embodiment. A ferromagnetic iron yoke 17 ofconical form for the front winding and an iron ferromagnetic yoke 18 ofannular form for the back winding may be placed on the outside of thewindings to increase the magnetic flux densities and hence increasemotor efficiency. The winding thicknesses should be designed for maximummotor efficiency, with the sum of their axial thicknesses somewhat lessthan but comparable to the magnet axial length. The yokes can be made ofsolid ferromagnetic material such as iron. To reduce “iron” losses, theyokes 17 can be laminated, for example in layers or by helically windingthin strip, or can be made of iron/powder epoxy composite. The yokesshould be positioned such that there is zero net axial magnetic force onthe impeller when it is positioned centrally in the housing. Themagnetic force is unstable and increases linearly with axialdisplacement of the impeller away from the central position, with thegradient being called the negative stiffness of the magnetic force. Thisunstable magnetic force must be countered by the hydrodynamic bearings,and so the stiffness should be made as small as possible. Choosing theyoke thickness such that the flux density is at the saturation levelreduces the stiffness and gives minimum mass. An alternative can be tohave no iron yokes, completely eliminating the unstable axial magneticforce, but the efficiency of such designs may be lower and the magneticflux density in the immediate vicinity of the pump may violate safetystandards and produce some tissue heating. In any case, the stiffness isacceptably small for slotless windings with the yokes present. Anotheralternative would be to insert the windings in slots in laminated ironstators which would increase motor efficiency and enable use of lessmagnet material and potentially lighter impeller blades. However, theunstable magnetic forces would be significant for such slotted motors.Also, the necessity for fat blades to generate the required bearingforces in this embodiment allows room for large magnets, and so slotlesswindings are chosen in the preferred embodiment.

[0103] Instead of determining the yoke positions so that the impellerhas zero magnetic axial force in the central position, it may bepossible to provide a bias axial magnetic force on the impeller, whichcan counteract other forces such as any average bulk hydrodynamic axialforce. In particular, by ensuring a net axial force into the conicalbody, the thrust bearings on the cover surface can be made superfluous.However, such a bias would demand greater average thrust forces, smallergaps and increased blood damage, and so the recommended goal is to zeroboth the magnetic and bulk hydrodynamic axial forces on the impellerwhen centrally positioned.

[0104] The overall design requirement for exclusive hydrodynamicsuspension requires control of the external force balance to make therelative magnitude of hydrodynamic thrust sufficient to overcome theexternal forces. Typical external forces include gravitational forcesand net magnetic forces arising as a result of the motor drive.

[0105] There are many options for the winding topology and number ofphases. FIG. 5A depicts the preferred topology for the body winding 15,viewed from the inlet axis.

[0106] The cover winding 16 looks similar but the coils need not avoidthe inlet tube and so they appear more triangular in shape. The bodywinding has a more complex three-dimensional shape with bends at theends of the body cone section. Each winding consists of three coils.Each coil is made from a number of turns of an insulated conductor suchas copper with the number of turns chosen to suit the desired voltage.The coil side mid-lines span an angle of about 50°-100° at the axis whenthe coils are in position. The coils for body and cover are alignedaxially and the axially adjacent coils are connected in either parallelor series connection to form one phase of the three phase winding.Parallel connection offers one means of redundancy in that if one coilfails, the phase can still carry current through the other coil. Inparallel connection each of the coil and body winding has a neutralpoint connection as depicted in FIG. 5A, whereas in series connection,only one of the windings has a neutral point.

[0107] An alternative three phase winding topology, depicted in FIG. 5B,uses four coils per phase for each of the body and cover windings, witheach coil wrapping around the yoke, a topology called a “Gramm ring”winding.

[0108] Yet another three phase winding topology, depicted in FIG. 5C,uses two coils per phase for each of the body and cover windings, andconnects the coil sides by azimuthal end-windings as is standard motorwinding practice. The coils are shown tilted to approximately follow theblade curvature, which can increase motor efficiency, especially for thephase energising strategy to be described below in which only one phaseis energised at a time. The winding construction can be simplified bylaying the coils around pins protruding from a temporary former, thepins shown as dots in 2 rings of 6 pins each in FIG. 5C. The coils arelabelled alphabetically in the order in which they would be layed, coilsa and d for phase A, b and e for phase B, and c and f for phase C.Instead of or as well as pins, the coil locations could be defined bythin fins, running between the pins in FIG. 5C, along the boundarybetween the coils. The coil connections depicted in FIG. 5C are thoseappropriate for the winding nearest the motor terminals for the case ofseries connection, with the optional lead from the neutral point on theother winding included.

[0109] The winding topologies depicted in FIG. 5B and C allow thepossibility of higher motor efficiency but only if significantly highercoil mass is allowed, and since option FIG. 5A is more compact andsimpler to manufacture, it is the preferred option. Material ribsbetween the coils of option FIG. 5A can be used to stiffen the housing.

[0110] Multi-stranded flexible conductors within a suitablebiocompatible cable can be used to connect the motor windings to a motorcontroller. The energisation of the three phases can be performed by astandard sensorless controller, in which two out of six semiconductingswitches in a three phase bridge are turned on at any one time.Alternatively, because of the relatively small fraction of the impellercross-section occupied by magnets, it may be slightly more efficient toonly activate one of the three phases at a time, and to return thecurrent by a conductor from the neutral point in the motor. Carefulattention must be paid to ensure that the integrity of all conductorsand connections is failsafe.

[0111] In the preferred embodiment, the two housing components 3 and 4are made by injection moulding from non-electrically conducting plasticmaterials such as Lexan polycarbonate plastic. Alternatively the housingcomponents can be made from ceramics. The windings and yokes are ideallyencapsulated within the housing during fabrication moulding. In thisway, the separation between the winding and the magnets is minimised,increasing the motor efficiency, and the housing is thick, increasingits mechanical stiffness. Alternatively, the windings can be positionedoutside the housing, of thickness at least around 2 mm for sufficientstiffness.

[0112] If the housing material plastic is hygroscopic or if the windingsare outside the housing, it may be necessary to first enclose thewindings and yoke in a very thin impermeable shell. Ideally the shellshould be non-conducting (such as ceramic or plastic), but titanium ofaround 0.1 mm to 0.2 mm thickness would give sufficiently low eddylosses. Encapsulation within such a shell would be needed to preventwinding movement.

[0113] Alternatively, the housing components 3 and 4 may be made from abiocompatible metallic material of low electrical conductivity, such asTi-6A1-4V. To minimise the eddy current loss, the material must be asthin as possible, e.g. 0.1 mm to 0.5 mm, wherever the materialexperiences high alternating magnetic flux densities, such as betweenthe coils and the housing inner surfaces 10 and 11 .

[0114] The combining of the motor and bearing components into theimpeller in the preferred embodiment provides several key advantages.The rotor consequently has very simple form, with the only cost of thebearing being tight manufacturing tolerances. The rotor mass is verylow, minimising the bearing force needed to overcome weight. Also, withthe bearings and the motor in the same region of the rotor, the bearingsforces are smaller than if they had to provide a torque to supportmagnets at an extremity of the rotor.

[0115] A disadvantage of the combination of functions in the impeller isthat its design is a coupled problem. The optimisation should ideallylink the fluid dynamics, magnetics and bearing thrust calculations. Inreality, the blade thickness can be first roughly sized to give adequatemotor efficiency and sufficient bearing forces with a safety margin.Fortuitously, both requirements are met for four blades of approximateaverage circumferential thickness 6 mm or more. The housing, blade, andsupport cone shapes can then be designed using computational fluiddynamics, maintaining the above minimum average blade thickness. Finallythe motor stator, i.e. winding and yoke, can be optimised for maximummotor efficiency.

[0116]FIG. 6 depicts an alternative embodiment of the invention as anaxial pump. The pump housing is made of two parts, a front part 19 and aback part 20, joined for example at 21. The pump has an axial inlet 22and axial outlet 23. The impeller comprises only blades 24 mounted on asupport cylinder 25 of reducing radius at each end. An important featureof this embodiment is that the blade edges are tapered to generatehydrodynamic thrust forces which suspend the impeller. These forcescould be used for radial suspension alone from the straight section 26of the housing, with some alternative means used for axial suspension,such as stable axial magnetic forces or a conventional tapered-land typehydrodynamic thrust bearing.

[0117]FIG. 6 proposes a design which uses the tapered blade edges toalso provide an axial hydrodynamic bearing. The housing is made with areducing radius at its ends to form a front face 27 and a back face 28from which the axial thrusts can suspend the motor axially. Magnets areembedded in the blades with blades having alternating polarity and fourblades being recommended. Iron in the outer radius of the supportcylinder 25 can be used to increase the magnet flux density.Alternatively, the magnets could be housed in the support cylinder andiron could be used in the blades. A slotless helical winding 29 isrecommended, with outward bending end-windings 30 at one end to enableinsertion of the impeller and inward bending windings 31 at the otherend to enable insertion of the winding into a cylindrical magnetic yoke32. The winding can be encapsulated in the back housing part 20.

[0118] Third Embodiment

[0119] With reference to FIGS. 7 to 15 inclusive there is shown afurther preferred embodiment of the pump assembly 200.

[0120] With particular reference initially to FIG. 7 the pump assembly200 comprises a housing body 201 adapted for bolted connection to ahousing cover 202 and so as to define a centrifugal pump cavity 203therewithin.

[0121] The cavity 203 houses an impeller 204 adapted to receive magnets205 within cavities 206 defined within blades 207. As for the firstembodiment the blades 207 are supported from a support cone 208.

[0122] Exterior to the cavity 203 but forming part of the pump assembly200 there is located a body winding 209 symmetrically mounted aroundinlet 210 and housed between the housing body 201 and a body yoke 211.

[0123] Also forming part of the pump assembly. 200 and also mountedexternal to pump cavity 203 is cover winding 212 located within windingcavity 213 which, in turn, is located within housing cover 202 andclosed by cover yoke 214.

[0124] The windings 212 and 209 are supplied from the electroniccontroller of FIG. 12 as for the first embodiment the windings arearranged to receive a three phase electrical supply and so as to set upa rotating magnetic field within cavity 203 which exerts a torque onmagnets 205 within the impeller 204 so as to urge the impeller 204 torotate substantially about central axis TT of cavity 203 and in linewith the longitudinal axis of inlet 210. The impeller 204 is caused torotate so as to urge fluid (in this case blood) around volute 215 andthrough outlet 216.

[0125] The assembly is bolted together in the manner indicated by screws217. The yokes 211, 214 are held in place by fasteners 218.Alternatively, press fitting is possible provided sufficient integrityof seal can be maintained.

[0126]FIG. 8 shows the impeller 204 of this embodiment and clearly showsthe support cone 208 from which the blades 207 extend. The axial cavity219 which is arranged, in use, to be aligned with the longitudinal axisof inlet 210 and through which blood is received for urging by blades207 is clearly visible.

[0127] The cutaway view of FIG. 9 shows the axial cavity 219 and alsothe magnet cavities 206 located within each blade 207. The preferredcone structure 220 extending from housing cover 202 aligned with theaxis of inlet 210 and axial cavity 219 of impeller 204 is also shown.

[0128]FIG. 10 is a side section, indicative view of the impeller 204defining the orientations of central axis FF, top taper edge DD andbottom taper edge BB, which tapers tare illustrated in FIG. 11 in sidesection view.

[0129]FIG. 11A is a section of a blade 207 of impeller 204 taken throughplane DD as defined in FIG. 10 and shows the top edge 221 to be profiledfrom a leading edge 223 to a trailing edge 224 as follows: centralportion 227 comprises an ellipse with centre on the dashed midlinehaving a semi-major axis of radius 113 mm and a semi-minor axis ofradius 80 mm and then followed by leading conical surface 225 andtrailing conical surface 226 on either side thereof as illustrated inFIG. 11A. The leading surface 225 has radius 0.05 mm less than thetrailing surface 226. This prescription is for a taper which can beachieved by a grinding wheel, but many alternative prescriptions couldbe devised to give a taper of similar utility.

[0130] The leading edge 223 is radiused as illustrated.

[0131]FIG. 11B illustrates in cross-section the bottom edge 222 of blade207 cut along plane BB of FIG. 10.

[0132] The bottom edge includes cap 228 utilised for sealing magnet 205within cavity 206.

[0133] In this instance substantially the entire edge comprises astraight taper with a radius of 0.05 mm at leading edge 229 and a radiusof 0.25 mm at trailing edge 230.

[0134] The blade 207 is 6.0 mm in width excluding the radii at eitherend.

[0135]FIG. 12 comprises a block diagram of the electrical controllersuitable for driving the pump assembly 200 and comprises a three phasecommutation controller 232 adapted to drive the windings 209, 212 of thepump assembly. The commutation controller 232 determines relative phaseand frequency values for driving the windings with reference to setpoint speed input 233 derived from physiological controller 234 which,in turn, receives control inputs 235 comprising motor current input andmotor speed (derived from the commutation controller 232), patient bloodflow 236, and venous oxygen saturation 237. The pump blood flow can beapproximately inferred from the motor speed and current via curve-fittedformulae.

[0136]FIG. 13 is a graph of pressure against flow for the pump assembly200 where the fluid pumped is 18% glycerol for impeller rotationvelocity over the range 1500 RPM to 2500 RPM. The 18% glycerol liquid isbelieved to be a good analogue for blood under certain circumstances,for example in the housing gap.

[0137]FIG. 14 graphs pump efficiency against flow for the same fluidover the same speed ranges as for FIG. 13.

[0138]FIG. 15 is a graph of electrical power consumption against flowfor the same fluid over the same speed ranges as for FIG. 13.

[0139] The common theme running through the first, second and thirdembodiments described thus far is the inclusion in the impeller of ataper or other deformed surface which, in use, moves relative to theadjacent housing wall thereby to cause a restriction with respect to theline of movement of the taper or deformity thereby to generate thrustupon the impeller which includes a component substantially normal to theline of movement of the surface and also normal to the adjacent internalpump wall with respect to which the restriction is defined for fluidlocated therebetween.

[0140] In order to provide both radial and axial direction control atleast one set of surfaces must be angled with respect to thelongitudinal axis of the impeller (preferably at approximately 45°thereto) thereby to generate or resolve opposed radial forces and anaxial force which can be balanced by a corresponding axial forcegenerated by at least one other tapered or deformed surface locatedelsewhere on the impeller.

[0141] In the forms thus far described top surfaces of the blades 8, 207are angled at approximately 45° with respect to the longitudinal axis ofthe impeller 100, 204 and arranged for rotation with respect to theinternal walls of a similarly angled conical pump housing. The topsurfaces of the blades are deformed so as to create the necessaryrestriction in the gap between the top surfaces of the blades and theinternal walls of the conical pump housing thereby to generate a thrustwhich can be resolved to both radial and axial components.

[0142] In the examples thus far the bottom faces of the blades 8, 207comprise surfaces substantially lying in a plane at right angles to theaxis of rotation of the impeller and, with their deformities define agap with respect to a lower inside face of the pump housing againstwhich a substantially only axial thrust is generated.

[0143] Other arrangements are possible which will also, relying on theseprinciples, provide the necessary balanced radial and axial forces. Sucharrangements can include a double cone arrangement where the conical topsurface of the blades is mnirrored in corresponding bottom conicalsurface. The only concern with this arrangement is the increased depthof pump which can be a problem for in vivo applications where sizeminimisation is an important criteria.

[0144] Fourth Embodiment

[0145] With reference to FIG. 18 a further embodiment of the inventionis illustrated comprising a plan view of the impeller 300 forming partof a “channel” pump. In this embodiment the blades 301 have been widenedrelative to the blades 207 of the third embodiment to the point wherethey are almost sector-shaped and the flow gaps between adjacent blades301, as a result, take the form of a channel 302, all in communicationwith axial cavity 303.

[0146] A further modification of this arrangement is illustrated in FIG.19 wherein impeller 304 includes sector-shaped blades 305 having curvedleading and trailing portions 306, 307 respectively thereby definingchannels 308 having fluted exit portions 309.

[0147] As with the first and second embodiments the radial and axialhydrodynamic forces are generated by appropriate profiling of the topand bottom faces of the blades 301, 305 (not shown in FIGS. 18 and 19).

[0148]FIG. 20 illustrates a perspective view of an impeller 304 whichfollows the theme of the impeller arrangement of FIGS. 18 and 19 inperspective view and where like parts are numbered as for FIG. 19. Inthis case the four blades 305 are joined at mid-portions thereof by ablade support in the form of a conical rim 350 and have edge portionswhich are shaped so as to have an increased curvature on the trailingedge 351 thereof compared with the leading edge 352.

[0149] Fifth Embodiment

[0150] A fifth embodiment of a pump assembly according to the inventioncomprises an impeller 410 as illustrated in FIG. 21 where, conceptually,the upper and lower surfaces of the blades of previous embodiments areinterconnected by a top shroud 411 and a bottom shroud 412. In thisembodiment the blades 413 can be reduced to a very small width as thehydrodynamic behaviour imparted by their surfaces in previousembodiments is now given effect by the profiling of the shrouds 411, 412which, in this instance, comprises a series of smooth-edged wedges withthe leading surface of one wedge directly interconnected to the trailingedge of the next leading wedge 414.

[0151] As for previous embodiments the top shroud 411 is of overallconical shape thereby to impart both radial and axial thrust forceswhilst the bottom shroud 412 is substantially planar thereby to impartsubstantially only axial thrust forces.

[0152] It is to be understood that, whilst the example of FIG. 21 showsthe surfaces of the shroud 411 angled at approximately 45° to thevertical, other inclinations are possible extending to an inclination of0° to the vertical which is to say the impeller 410 can take the form ofa cylinder with surface rippling or other deformations which impart thenecessarily hydrodynamic lift, in use.

[0153] With reference to FIGS. 22 to 24 a specific example of theconcept embodied in FIG. 21 is illustrated and wherein like componentsare numbered as for FIG. 21.

[0154] It will be observed that, with reference to FIG. 24, the blades413 are thin compared to previous embodiments and, in this instance, areactuate channels 416 therebetween which allow fluid communication from acentre volume 417 to the periphery 418 of the impeller 410.

[0155] In this arrangement it will be noted that the wedges 414 areseparated one from the other on each shroud by channels 419. Thechannels extend radially down the shroud from the centre volume 417 tothe periphery 418.

[0156] In such designs with thin blades, the magnets required for thedriving torque can be contained within the top or bottom volute or both,along with the optional soft magnetic yokes to increase motorefficiency.

[0157] A variation of this embodiment is to have the wedge profiling cutinto the inter surfaces of the housing and have smooth shroud surfaces.

[0158] Sixth Embodiment

[0159] In contrast to the embodiments illustrated with respect to FIGS.3A, 3B and 3C an arrangement is shown in FIG. 25 wherein the “deformedsurface” comprises a stepped formation 510 forming part of an inner wallof the pump housing (not shown). In this instance the rotor includingblade 511 includes a flat working surface 512 (and not having a deformedsurface therein) which is adapted for relative movement in the directionof the arrow shown with respect to the stepped formation 510 thereby togenerate hydrodynamic thrust therebetween.

[0160] Seventh Embodiment

[0161] With reference to FIG. 26 there is shown an arrangement of rotorblade 610 with respect to stepped formation 611 and wherein the rotorblade 610 includes a deformed surface 612 at a working face thereof. Inthis instance the deformation comprise curved edges 613, 614. As for theprevious embodiment relative movement of the rotor blade 610 in thedirection of the arrow with respect to deformed surface 611 forming partof the pump housing (not shown) causes relative hydrodynamic thrusttherebetween.

[0162] The foregoing describes principles and examples of the presentinvention, and modifications, obvious to those skilled in the art, canbe made thereto without departing from the scope and spirit of theinvention.

[0163] Principles of Operation

[0164] With particular reference to FIG. 27 this specification describesthe suspension of an impeller 600 within a pump housing 601 by the useof hydrodynamic forces. In this specification the suspension of theimpeller 600 is performed dominantly which is to say exclusively byhydrodynamic forces.

[0165] The hydrodynamic forces are forces which are created by relativemovement between two surfaces which have a fluid in the gap between thetwo surfaces. In the case of the use of the pump assembly 602 as arotary blood pump the fluid is blood.

[0166] The hydrodynamic forces can arise during relative movementbetween two surfaces even where those surfaces are substantiallyentirely parallel to each other or non-deformed. However, in thisspecification, hydrodynamic forces are caused to arise during relativemovement between two surfaces where at least one of the surfacesincludes a “deformed surface”.

[0167] In this specification “deformed surface” means a surface whichincludes an irregularity relative to a surface which it faces such that,when the surface moves in a predetermined direction relative to thesurface which it faces the fluid located in the gap there betweenexperiences a change in relative distance between the surfaces along theline of movement thereby to cause a hydrodynamic force to arisetherebetween in the form of a thrust force including at least acomponent substantially normal to the plane of the gap defined at anygiven point between the facing surfaces.

[0168] In the example of FIG. 27 there is a first deformed surface 603forming at least part of a first face 609 of impeller 600 and a seconddeformed surface 605 on a second face 606 of the impeller 600.

[0169] The inset of FIG. 27 illustrates conceptually how the firstdeformed surface 603 may form only part of the first face 604.

[0170] The first deformed surface 603 faces first inner surface 607 ofthe pump housing 601 whilst second deformed surface 605 faces secondinner surface 608 of the pump housing 601.

[0171] In use first gap 609 defined between first deformed surface 603and first inner surface 607 has a fluid comprising blood located thereinwhilst second gap 610 defined between second deformed surface 605 andsecond inner surface 608 also has a fluid comprising blood locatedtherein.

[0172] In use impeller 600 is caused to rotate about impeller axis 611such that relative movement across first gap 609 between first deformedsurface 603 and first inner face 607 occurs and also relative movementacross second gap 610 between second deformed surface 605 and secondinner surface 608 occurs. The orientation of the deformities of firstdeformed surface 603 and second deformed surface 605 relative to theline of movement of the deformed surfaces 603, 605 relative to the innersurfaces 607, 608 is such that the fluid in the gaps 609, 610experiences a change in height of the gap 609, 610 as a function of timeand with the rate of change dependant on the shape of the deformities ofthe deformed surfaces and also the rate of rotation of the impeller 600relative to the housing 601. That is, at any given point on either innersurface 607 or 608, the height of the gap between the inner surface 607or 608 and corresponding deformed surface 603 or 605 will vary with timedue to passage of the deformed surface 603 or 605 over the innersurface.

[0173] Hydrodynamic forces in the form of thrust forces normal to theline of relative movement of the respective deformed surfaces 603, 605relative to the inner surfaces 607, 608 thus arise.

[0174] With this configuration it will be noted that the first gap 609lies substantially in a single plane whilst the second gap 610 is it theform of a cone and angled at an acute angle relative to the plane of thefirst gap 609.

[0175] Accordingly, the thrust forces which can be enlisted to first gap609 and second gap 610 are substantially normal to and distributedacross both the predominantly flat plane of first deformed surface 603and normal to the substantially conical surface of second deformedsurface 605 thereby permitting restoring forces to be applied betweenthe impeller 600 and the pump housing 601 thereby to resist forces whichseek to translate the impeller 600 in space relative to the pump housing601 and also to rotate the impeller 600 about any axis (other than aboutthe impeller axis 611) relative to the pump housing 601. Thisarrangement substantially resists five degrees of freedom of movement ofimpeller 600 with respect to the housing 601 and does so predominantlywithout any external intervention to control the position of theimpeller with respect to the housing given that disturbing forces, fromother sources, most notably magnetic forces on the impeller due to itsuse as rotor of the motor are net zero when the impeller occupies asuitable equilibrium position. The balance of all forces on the rotor,effected by manipulation of magnetic and other external sources, may beadjusted such that the rotor is predominantly hydrodynamically born.

[0176] It will be observed that these forces increase as the gaps 609,610 narrow relative to a defined operating position and decrease as thegaps 609, 610 increase relative to a defined operating gap. Because ofthe opposed orientation of first deformed surface 603 relative to seconddeformed surface 605 it is possible to design for an equilibriumposition of the impeller 600 within the pump housing 601 at a definedequilibrium gap distance for gaps 609, 610 at a specified rotorrotational speed about axis 611 and rotor mass leading to a closeapproximation to an unconditionally stable environment for the impeller600 within the pump housing 601 against a range of disturbing forces.

[0177] Characteristics and advantages which flow from the arrangementdescribed above and with reference to the embodiments includes:

[0178] 1. Low haemolysis, hence low running speed and controlled fluiddynamics (especially shear stress) in the gap between the casing andimpeller. This in turn led to the selection of radial off-flow andminimal incidence at on-flow to the rotor;

[0179] 2. Radial or near-radial off-flow from the impeller can be chosenin order to yield a “flat” pump characteristic (HQ) curve.

INDUSTRIAL APPLICABILITY

[0180] The pump assembly 1, 200 is applicable to pump fluids such asblood on a continuous basis. With its expected reliability it isparticularly applicable as an in vivo heart assist pump.

[0181] The pump assembly can also be used with advantage for the pumpingof other fluids where damage to the fluid due to high shear stressesmust be avoided or where leakage of the fluid must be prevented with avery high degree of reliability—for example where the fluid is adangerous fluid.

What is claimed is:
 1. A rotary blood pump for use in a heart assistdevice or like device, said pump having an impeller suspended in usewithin a pump housing exclusively by hydrodynamic thrust forcesgenerated by relative movement of said impeller with respect to andwithin said pump housing.